Applicant is aware of the following U.S. Patents.
______________________________________ 2,185,987 2,938,786 3,758,294 3,758,296 3,813,239 3,817,747 3,844,774 3,892,541 3,893,851 4,033,767 ______________________________________
The disclosures of the above listed patents are incorporated by reference herein.
Equipment used in highly corrosive environments typically is constructed of metal alloys such as stainless steel or other high alloys. These alloys are necessary to withstand the extremely corrosive effects of environments in which the equipment encounters chemicals such as concentrated sulfuric acid or concentrated phosphoric acid. A particularly difficult environment is encountered in making phosphate fertilizer. In the digestion of phosphate rock with hot, concentrated sulfuric acid, equipment must resist the environment at temperatures up to about 100.degree. C. The impure phosphoric acid which is produced can be extremely corrosive and contains some residual sulfuric acid. The corrosive effect is often increased by other impurities in the phosphoric acid, particularly by halogen ions such as chloride and fluoride, which are normally present in the phosphate rock feedstock used in the process. An extremely corrosive environment is encountered in the concentration of the crude phosphoric acid.
Applicants have produced a new alloy which has particular corrosion resistance in the environment encountered in producing phosphate fertilizer. In addition to superior corrosion resistance, the new alloy is relatively inexpensive and is highly castable to form complex parts and shapes. The alloy may be prepared by conventional and inexpensive air melt techniques, which is a particular advantage. Applicants' alloy typically contains between about 20-25% chromium, 6-9% molybdenum, 0.5-1% silicon, 2-4% manganese, 15-20% iron, 4-8% cobalt, up to 0.2% nitrogen, up to 0.2% carbon and less than about 0.15% copper; a low copper content is preferred. The balance (about 33-53%) is nickel.
Applicants' alloy is an air melted, substantially copper free, nickel base corrosion resistant alloy. Applicant has discovered, contrary to conventional wisdom, that an essentially copper free alloy exhibits corrosion resistance equal to and in most instances significantly better than similar alloys containing copper, particularly in the severe environment encountered in the concentration of phosphoric acid for fertilizers. This is particular true where quantities of halogen ions, as chloride and fluoride, are present.
Applicants have discovered that their particular substantially copper free alloys are significantly superior to commerical alloys normally used in this service, such as Hasteloy C276. Applicants' alloys have the significant advantage that they may be formed by standard air melting techniques and do not required the special techniques required by conventional high alloys used in this service, such as vacuum or electroslag processing. High alloys requiring such low carbon and silicon residuals must be melted using specialized melting techniques and are generally available only in wrought form. They cannot be produced by casting in commercial foundries using air melting techniques.
The very low carbon and silicon contents which are specified for the commercial high alloys are produced by these expensive melting techniques. It is known that a relatively high silicon content promotes fluidity of the molten metal and renders the melt castable. At the extremely low silicon content specified for the high alloys, the molten metal lacks fluidity and cannot be cast by conventional sand, investment or centrifugal foundry methods.
It is generally known that copper content in corrosion resistant alloys, such as the austentic stainless steels and certain other high nickel alloys, enhances the corrosion resistance of these alloys in environments where the alloys are exposed to acids of sulfur and phosphorus. Typical corrosion resistant alloys make use of a significant copper content to achieve better corrosion resistance. It is known that if the copper content is too high, it can cause a condition known as hot shortness in the alloys which makes them difficult to cast or hot work. Copper also may reduce the weldability of these alloys, but conventionally, significant copper content is desirable. Applicant's have found, however, that they can product a highly corrosion resistant alloy which is essentially copper free. In doing so, applicants also have produced an alloy which is weldable, which can result in high process yields and in a reduction of scrap and waste metal. These factors all contribute to a much lower product cost in applicants' alloy.
Phosphate rock deposits at various locations in the world vary greatly in chemical composition. The most severe corrosion environments are typically encountered in processing deposits of phosphate rock which contain a high content of halogens, such as chloride or fluoride. It is an object of applicants' invention to produce a material of construction suitable for use in processing such phosphate rock which presents a severely corrosive environment.
It is also an object of applicants' invention to produce a corrosion resistant alloy which is low in copper and which has an enchanced corrosion resistance.
It is a further object of applicants' invention to produce a highly corrosion resistant alloy which contains silicon in sufficient quantity to render the alloy castable by conventional methods.
It is an object of applicants' invention to produce a highly corrosion resistant alloy which contains silicon.
It is an object of applicants' invention to produce a corrosion resistant alloy that is essentially copper free.
It is an object of applicants' invention to produce a corrosion resistant alloy which has high strength and hardness properties.
Applicants' substantially copper free alloy may be made in two forms, depending upon the level of carbon in each form. The ultra low carbon alloys of applicants' invention have a carbon content of less than about 0.08% and have an austenitic solid solution structure when solution treated. The low carbon alloys, with a carbon content of between about 0.10 and 0.20%, exhibit a precipitation of a Chinese script configuration. It will be understood that, as used herein, the terms "low carbon" and "ultra low carbon" are meant to describe alloys having the above carbon contents. The precipitates have been identified as heavy metal carbides. The micro hardness test, converted to Rockwell C scale, shows a matrix hardness in the low carbon alloy matrix of about 26.7 and about 52.3 hardness in the carbide. The low carbon alloys do not have the exceptionally high corrosion resistance exhibited by the ultra low carbon alloy. However, the low carbon alloys have a structure which may be highly useful in corrosive services where physical abrasion, erosion or galling is encountered.
The invention may be further understood by reference to the following Description of the Preferred Embodiments.